Narrow linewidth tunable lasers are fundamental components to experiments involving spectroscopy, cooling and trapping atoms, and quantum information science where they are often used to address atomic and molecular energy transitions. Since their introduction in the 1960s, diode laser systems have been consistently space-efficient and cost-effective tools for such experimentation. However, their intrinsic linewidths normally range from hundreds of megahertz to a few gigahertz, limiting their applications in precision spectroscopy and the control of narrow atomic transitions.
Some of the first efforts to narrow the linewidths of diode lasers yielded extended cavity diode lasers (ECDLs), distributed Bragg reflector (DBR) lasers, and fiber lasers. In general, ECDLs narrow the linewidth of a diode laser by using a diffraction grating to create an external cavity that amplifies a precise and stable wavelength of light. Linewidth reduction has also been demonstrated with feedback from a resonator as well as in distributed feedback (DFB) and DBR lasers. Adding passive external fiber optical feedback to a DBR laser yields sub-kilohertz linewidth narrowing in DBR lasers and traditional ECDLs. Active mechanisms for noise reduction in diode lasers include servo-electronic-based stabilization to a high-finesse ultra-stable cavity or Michelson interferometer, but the finite bandwidth of electrical feedback to the diode current or temperature makes it difficult to reduce high frequency noise.